if you got photoshop, save a texture from doom and load it up in photoshop. then select image > mode > indexed

then image > mode > color table

you should see the pallete for doom. save the pallete. then load up your titlepic and select image > mode > colortable and load the pallete you saved. also be sure the titlepic is indexed too.
the pic will then have the colors from Doom.

To save a palette: Colors, Save Palette, give it a name - like DOOM.PAL

To have it use the palette and attempt to match the colors: Colors, Load Palette (DOOM.PAL file created). There are 3 days to match. Normally play with the 1st 2 and see which one you like best. If you are doing a lot, use the Shift+O shortcut key.

Not all conversions work that well though, if you started with some other palette or 16/32 bit color images.

The problem is that DooM uses 256-colour bitmaps, and the one you have is probably a 24-bit bitmap. (and as you know, if you switch from 24-bit to 256-colour, it goes all crappy) If it was a photo then that's definitely the case. I have that trouble with converting Quake 2 screenshots to DooM. Sometimes it works out fine, but other times it's a pain. I don't really know how to tackle it, as you have to lose all those extra colours, which will always mess up the pic.
If you ever want to convert screenshots, the best thing to do is take them in a low resolution with software rendering.

Yeah, my preferred method of getting things to Doom is loading the palette on "ERROR DIFFUSION" mode...however, it almost looks worse than if I did "NEAREST COLOR" because of misplaced speckles everywhere. For an example, look at the AK47 view sprites in Weasel's Weapons...it's a good sprite except for those speckles! EEW.

A good way to transfer a picture into the Doom palette, at least in my experience:

* Have picture (duh)
* Load the Doom palette using "error diffusion dithering"
* Bring the picture back up to 24-bit
* Using whatever tool you like best, go back and touch up the speckles. I use PSP and I use the magic finger "smooth" or "smudge" tool to blur the speckles into their surroundings.
* Load the Doom palette with "nearest color matching"

Or, do the palettes in the reverse order. Whatever looks nicer. Doing that, you should be able to get a decent looking picture in the Doom palette.